The present invention relates to a conveyor having self-propelled carriers, for example, for use in motor vehicle production lines.
With motor vehicle production lines, a plurality of carriers having vehicle parts (article to be transported) placed thereon need to be moved through a specified work zone with a constant distance maintained between the carriers. Accordingly, power-and-free trolley conveyors have heretofore been used.
However, the power-and-free trolley conveyor employs chains and therefore has the problem of giving off a great noise and being unable to drive the carriers at a high speed in transport zones or the like other than the work zone.
Accordingly, conveyors having a plurality of self-propelled carriers have been proposed to ensure a reduced noise and to drive the carriers at a higher speed.
The self-propelled carrier of the conventional conveyor of the first-mentioned type has one drive wheel rollable on a rail and one electric motor for driving the wheel.
The conveyor therefore has the problem that when the rail has a gradient, the drive wheel slips at the gradient portion, making the carrier unable to run smoothly.
To overcome this problem, it has been proposed to provide a carrier lift between a high horizontal rail portion and a lower horizontal rail portion without giving the gradient to the rail, or to provide a chain conveyor for the gradient portion to move the carrier with the chain (see Unexamined Japanese Patent Publication SHO 52-97573). However, the lift or chain conveyor needed results in an increased equipment cost.
A conveyor is also proposed wherein a rack having upward teeth is provided at a rail gradient portion, and a pinion mounted on the same shaft as the drive wheel of a carrier is adapted to roll on the rack at the gradient portion (see Examined Japanese Patent Publication SHO 52-97575). However, this arrangement has a problem in respect of strength since a load acts on the pinion at the shaft end of the drive wheel. Further because the pinion as positioned on the rack is subjected to the load of the carrier, the pinion fails to mesh with the rack smoothly, while these members undergo marked abrasion.
Further in the case where the carrier is adapted to run selectively at a high speed or a low speed, the single motor conventionally used for driving the drive wheel encounters the problem of an insufficient torque when driving the carrier at the low speed.